In recent years, studies have been vigorously made on an organic thin-film light emitting device that emits light when electrons injected from a cathode and holes injected from an anode are recombined inside an organic fluorescent body sandwiched between the two electrodes. This light emitting device is characterized by a thin structure, high luminance light emission under a low driving voltage, and light emissions with multiple colors achieved by selecting fluorescent materials, and has drawn public attentions.
These studies have been carried out by many research organizations since C. W. Tangs, et al of Kodak Company indicated that an organic thin-film device could emit light with high luminance. The typical structure of the organic thin-film light emitting device, proposed by the research group of Kodak Company, was prepared by successively stacking a hole transporting diamine compound, 8-hydroxyquinoline aluminum serving as an emissive layer, and Mg:Ag serving as a cathode on an ITO glass substrate, and green light emission of 1,000 cd/m2 was available at a driving voltage of about 10 V (see Non-Patent Document 1).
Moreover, since the organic thin-film light emitting device allows many luminescent colors to be obtained by using various kinds of fluorescent materials for the emissive layer, studies for putting the device into practical use for displays and the like have been progressively carried out. Among the emissive materials for the three primary colors, studies for green color emissive materials have been developed most greatly, and at present, intensive studies have been carried out on red color emissive materials and blue color emissive materials so as to improve their characteristics.
The organic thin-film light emitting device needs to be improved in luminance efficiency, reduced in their driving voltage, and also improved in durability. Among these, in the case when the luminance efficiency is poor, an image output required for high luminance is not available to cause high power consumption in outputting an image with desired luminance. In order to improve the luminance efficiency, various emissive materials have been developed (for example, see Patent Documents 1 to 5). Moreover, a technique for doping a material to be used as an electron transporting layer with an alkali metal has been proposed (see Patent Documents 6 to 10).